Recently a new CC chemokine has been cloned and characterized, macrophage derived chemokine (MDC). MDC is not closely related to other known chemokines and is produced by macrophages and dendritic cells. It is chemotactic for monocytes, monocyte-derived dendritic cells, and IL-2-activated natural killer cells (Godiska et al, J. Exp. Med., 185(9):1595 (1997)).
It has been clearly demonstrated that certain factors produced by activated CD8+ T cells have been implicated in the suppression of HIV-1 infection (Walker et al., Science, 234, 1563, 1986; Brincham et al., J. Immunol. 144, 2961, 1990). The production of this suppressor activity correlates with immune status and shows a steady decline in parallel with HIV disease progression (Blackbourn et al., PNAS 93, 13125,1996; Mackewicz et al., J. Clin. Invest., 87, 1462, 1991). Although chemokines RANTES, MIP-1a and MIP-1b were shown to be responsible for CD8 suppressor activity (Cocchi et al., Science 270, 1811, 1995), these chemokines were not able to explain the full complement of suppressor activity mediated by CD8+ T cells. Recently, Pal et al. have identified a protein from the culture supernatant of HTLV-I transformed CD8+ T lymphocytes which was shown to suppress infection by different phenotypes of HIV-1. This protein was identified to be a variant of MDC as described by Godiska et al. except for the N-terminal amino acid, a difference that is presumably due to variability in NH.sub.2 terminal processing between cell types (Pal et al, Science, 278:695 (1997) and U.S. patent application Ser. No. 08/931,764, which is herein incorporated by reference).
The super family of chemoattractant cytokines (chemokines) and their receptors are involved in inflammation and infection. The chemokines range in size from 68 to 120 amino acids (in the mature form) and can be divided into two major classes based on variations in a shared cysteine motif. The largest group, the C--C, or .beta. chemokines has nearly 20 members identified to date. The C-X-C, or .alpha. chemokine branch can be further subdivided into two groups based on structure and function. The largest of these groups contains proteins containing the E-L-R-C-X-C motif and the smaller group is made up of proteins without the E-L-R amino terminal to C-X-C.
The structural classes parallel function to a large extent in that most C-X-C chemokines are chemoattractants for neutrophils but not monocytes, whereas C--C chemokines generally attract monocytes, T-lymphocytes, and in some cases eosinophils, basophils, or mast cells.
The repertoire of known human CC chemokines is expanding rapidly and now includes MIP-1.alpha., MIP-1.beta., RANTES, I-309, monocyte chemotactic proteins 1, 2, and 3 (MCP-1, -2, -3), MCP-4, eotaxin, HCC-1, thymus and activation regulated chemokine TARC), and Exodus. These proteins are 70-100 amino acids long and have 25-70% identity with each other.
Chemokines act through G-protein coupled receptors, which have a characteristic seven-transmembrane structure. These proteins are structurally related, with amino acid homology high in the transmembrane regions and some intracellular loops. There seems to be less homology at the N- and C-termini, and extracellular loops, which are presumed to be involved in ligand binding (N-terminal) and receptor specific interactions with signaling components.
Five CC chemokine receptors have been described: CCR-1 binds MIP-1.alpha., RANTES, and MCP-3; CCR-2 binds MCP-1, MCP-3, and MCP-4; CCR-3 binds eotaxin, MCP-3, RANTES, and MCP-4; CCR-4 binds MIP-1.alpha., RANTES, and MCP-1; and CCR-5 binds MIP-1.alpha., MIP-1.beta., and RANTES. Macrophage-tropic strains of HIV appear to require one of the receptors, primarily CCR-5, as a cofactor for infection.
Since MDC contributes a significant portion of the suppressor activity mediated by CDF8+ T cells, identification and quantitation of MDC gene may serve as a prognostic indicator of HIV-1 infection. In particular, it may be necessary to determine whether the level of MDC gene is correlated with virus load in HIV-1 infected individuals. Furthermore, induction of the level of MDC gene in vaccinated subjects may determine the effectiveness of the test vaccine against AIDS. Therefore, establishment of assays to detect and quantitate MDC RNA should serve as an important tool to study pathogenesis in HIV-1 infected patients.